The invention concerns a method of sifting particulate material in a cross-current at cut sizes considerably below 1 mm, and apparatus for carrying out cross-current sifting with a high throughput and satisfactory sharpness of separation of very fine powders. It concerns more particularly a further development and improvement of the cross-current methods and apparatus, described in U.S. Pat. Nos. 3,311,234 and 3,520,407 and in German Pat. Nos. 1,507,736 and 1,607,656 as known from Applicant's German Patent Specification No. 1,482,458 (corresponding U.S. Patent Specification No. 3,311,234); 1,507,735 (corresponding U.S. Patent Specification No. 3,520,407); and 1,507,736 and 1,607,656.
In these known cross-current air sifting methods of separating particulate material into two or more fractions, all particles of the same size and charged i.e. propelled with the same velocity of at least 5 m/sec in the same direction in a thin layer transversely into a sifting gas current of high velocity, precluding any decisive influence of gravity, are spread out in the current and after a time of flight of the order of magnitude of 1/100 sec are separated into two or more fractions by one or more knife edges opposing the particles trajectories without previous impingement on any wall. While the fine material is entrained by the sifting gas current into a flow duct, the coarse material passes over the knife edge into a coarse material collecting receptacle. The sifting-gas current may be plane as disclosed in U.S. Pat. No. 3,311,234, so that all particles of the material enter in a thin plane layer into a plane flow, whose flow planes, agreeing with the movement planes of the particles, possess congruent velocity fields, the flow velocity of the sifting-gas current being at least 20 m/sec in order to preclude the influence of gravity. The sifting gas current on the coarse material output side, opposite the material input side, has a free jet boundary through which the coarse particles enter a coarse material collecting receptacle. The sifting gas current entraining the fine particles can be divided at particle track surfaces into at least two fractions, from which the fine particles can be separately exhausted from one or the other fraction. At the same time, the oncoming sifting gas current, downstream of the material inlet point, can be divided into at least two partial currents which are charged in each case with fine or very fine material and which are initially led off separately at a distance from the material inlet parallel to the inlet flow direction. The sifting gas current may, however, also have an axially-symmetrical annular cross-section, into which the particles are introduced from the inside in a thin layer by means of a centrifugal plate as described in U.S. Pat. No. 3,520,407.
Sifting may be carried out in each case under pressure or suction, i.e. a blower producing the sifting gas current may force the sifting gas into the sifting zone, or exhaust it from the said zone.
In a cross-stream i.e. a cross current or transverse sifter for carrying out the plane sifting process, the material is fed by a conveyor into a sifting gas current carried before and after the sifting zone in a flow duct. The duct walls are interrupted in the region of the material inlet and the coarse material outlet is situated inlet. The discharge roller of a conveyor belt feeder is located outside the flow duct. The sifting gas current enters the sifting zone through a nozzle immediately in front of the material inlet with the same velocity over its entire cross section. A flow straightener may be provided preceeding the nozzle. The sifting zone includes an adjustable knife edge facing the flow at the coarse material outlet which forms the boundary of the sifting zone. The conveyor belt for introducing the material, on the side running towards the material inlet, may be covered over the entire belt width or preferably over the middle part of the belt width by another conveyor belt moving at the same speed and spaced, at the most, at a slight distance above it. The known cross-current sifter for carrying out the axially-symmetrical cross-current sifting method has a stationary flow duct for sifting gas charged with fine material, a centrifugal plate situated at the duct inlet and co-axial with it, a coarse-material collecting receptacle surrounding the duct, and a sifting gas annular nozzle, preceding the centrifugal plate and flow duct co-axially with axial spacing and tapering in the direction of flow towards the inlet orifice. The side of the centrifugal plate contacted by the sifted material has, at least in the radially outer region, the shape of a concave-conical or concave-curved surface of revolution. It is covered at a slight distance by a cover extending to the outer edge of the plate. The external diameter of the centrifugal plate is almost equal to or smaller than the inner diameter of the flow duct. The inlet for the coarse material projected by the centrifugal plate into the coarse material collecting receptacle is coaxial with the duct inlet. Its diameter is no larger than the external diameter of the duct inlet. The annular nozzle for the sifting gas may be preceded by a flow straightener. The sifting gas current passes outside the external diameter of the centrifugal plate. The sifting gas current has along the periphery of the nozzle outlet an equally high velocity and at the nozzle outlet is directed parallel to the axis of the flow duct into the duct inlet. On the flow-duct outer wall facing the nozzle is an axially slidable cylindrical knife edge which limits the sifting zone and over which the coarse material passes into the coarse material collecting receptacle. The inner wall of the flow duct extends cylindrically in the direction of the sifting gas flow at the outlet from the sifting gas nozzle.
The present invention is based on this cross-current sifting method and apparatus. It is thus to be differentiated from all separated methods and devices in which gravity plays a part. This will be seen from the following table which for material particles of density 1 g/cm.sup.3 gives the distance of fall in air in 1/100 sec. ##EQU1##
Above 300.mu., the distance of fall in 1/100 sec is as yet unaffected by air friction and amounts to 0.5 g t.sup.2 = 0.5 mm, and at 3/100 sec to 4.5 mm. Owing to gravity, therefore, the dispersion range of any particle distribution is only 0.5 mm or at 3/100 sec, which may be regarded as the upper limit of the time of flight for the cross-current method of separation of the present type, only 4.5 mm. In such a case no technical wind sifting is possible. In the case of a method of this type, gravity is actually without influence. The dispersion of the material is affected only the the sifting gas flow. Separation is thus independent of the absolute direction of movement of the material and the sifting gas flow in space, but on how the flow is directed with respects to the input of material and how high the velocity contributions are. In principle, the input of material may be from above downward, from below upward, horizontally or obliquely. The length of each material trajectory from input to the knife edge at 1/100 sec time of flight and with a 10 m/sec input velocity is 10 cm, at 20 m/sec input velocity it is 5 cm, at 2/100 sec and 10 m/sec input velocity, it is 20 cm. Much longer flight paths, i.e. more than 0.5 m, could be desirable, but are scarcely compatible with cross-current wind sifting of this type.
Cross-current sifting of this type thus differs unmistakably from the known whirlwind e.g. cyclone and the like sifting methods, in which the material is projected from a rotating plate into an ascending current and the fine material is carried out at the top, while the coarse material descends. In such methods, gravity is always largely involved. Insofar as applies to the separation, the sifters are not cross-current wind sifters but counter-current equlibrium wind sifters with gravity separation. Furthermore, in these known whirlwind air sifters, the sifting zone extends as far as the cylindrical boundary wall of the ascending sifting currents that is to say, to the flow duct wall. If the coarsest material is not previously sedimented out and the finest material is entrained upwardly by the current, the material strikes against the wall and is then subjected to renewed separating conditions. In all commercially important whirlwind air sifters, a rotating flow component is superimposed on the ascending air flow. The scattering plate then mainly has the function of distributing the material in the ascending air flow. It does not yet determine even the velocity of the material essential for separation. On the contrary, this is effected by the centrifugal force in the rotating flow. In this, method the processes taking place directly on the flow-duct wall, for example the rotary impulse exchange and secondary air flow occurring there have a substantial influence on the separating effect. In these whirlwind sifters, therefore, the zone of separation extends as far as the cylindrical duct wall which in commercial sifters of over 2m in diameter is far more than 0.5 m from the rotating feed plate circumference. It extends farther down, where the air flows opposite to the decending coarse material, and often extends by much more than 1 m upwardly, where centrifugal sifting of the material in the ascending flow continues. In centrifugal wind sifters, the ascending flow often receives an inwardly directed flow component, so that centerflow equilibrium sifting is produced for sifting out the fine material from the coarser sprayed particles.
In the cross-current type of sifting method, on the contrary, separation is effected as a cross-current separation which, due to the high input speed of the material, is conditioned into a rapid sifting-gas flow of low width the flow velocity having to be so high that the material, in a flight time of the order of magnitude of 1/100 sec is fanned out enough so that it can be separated into fractions by the knife edges opposing the trajectories of the material. Separation takes place in free flight and is not affected by rebound of the material trajectories on a wall, except for the unavoidable rebound of a trajectory which exactly hits the separating limit at a knife.
In the cross-current method discussed in the foregoing, the entire sifting gas, entering the separating zone through a nozzle provided with flow straightener and possibly an annular additional nozzle and/or an annular additional gas inlet provided outside the nozzle, enters the flow duct and carries the fine material contained in the charged material along with it. The coarse material then flies through the sifting gas current over a knife edge situated on the side of the flow duct opposite the material inlet, and enters the coarse material collecting chamber or receptacle. In a further modification of the basic method the fine material separated by the outer knife edge at the flow duct edge and the flowing sifting gas are subsequently separated by a further knife edge arranged for example centrally in the flow duct, into two fractions and two partial currents. In this case, however, part of the material before separation rebounds on the side of the outer knife edge and can ebound over the central knife edge into the inner flow duct. To this extent, this subsequent separation does not come within the type of method discussed here. On the other hand, even in this modification, the direction of flow of both departing partial currents is the same as the direction of the arriving flow.
A particularly favourable effect of cross-current sifting of this type has been found to be that even in the case of large quantities of the charged material, it separates sharply, and above all separation is shifted to fine separation limits. Thus, it is possible in an axially-symmetrical cross-current sifter according to U.S. Pat. No. 3,520,407, in which the flow duct has an annular inlet for the sifting gas charged with fine material, in a separating zone of 30 cm internal diameter and 38 cm external diameter, i.e. about 4 cm radial extent -- which corresponds to a path of flight of about 6cm in length -- for 10 ton /h material input quantity, to attain a sharp separation at 9.mu. separating limit. For smaller quantities of material, very sharp separations are certainly possible, but the separation limit is much higher. It is characteristic of the known cross-current wind shifting method that separation is independent of charging only up to a certain charge rate of material, and the separation limit cannot be adjusted below a certain value. Thus in the axially-symmetrical cross-current wind sifter, which gave the 9.mu. separation limit at 10 ton/h, even under extreme conditions, namely above 70 m/sec material input velocity, only about 6 cm flight path (flying time below 1/1000 sec) and only 20 m/sec air velocity, the separation limit between coarse-material and fine material, in a range of operation which is independent of charge rate, cannot be reduced below 40.mu.. Only by increasing the quantity of material beyond a certain limit, has it been possible to shift the separation limit to smaller particle sizes. At the same time, however, the separation sharpness is reduced; it is possible, however, to obtain still sufficiently sharp separations until the charge of material exceeds by more than ten times the limit of the charge-independent range.
The range of separation which is independent of charge rate is commercially very interesting because it permits sifting of large quantities of material, as well as very fine separation.
The problem underlying the present invention is to provide a method and device for cross-current sifting of particulate material at separation limits below 1 mm, in particular below 300.mu. down to a few .mu., in which separation is largely independent of the material charge-rate, and also, in the charge-rate independent stable separation range, permits much lower separation limits to be attained than heretofore, while at the same time ensuring the principal advantage of cross-current sifting of this type, i.e. of attaining a high sharpness of separation even for exceptionally high material charge rates of the sifting current and heavy throughputs.